The present invention is directed generally to a process for making synthesis gas from which streams of carbon monoxide and methanol can be obtained for the manufacture of acetic acid, and more particularly to the retrofit of a methanol plant to divert all or a portion of the syngas from the existing methanol synthesis loop to a carbon monoxide separator and to react the methanol from the methanol synthesis loop with the carbon monoxide from the separator in approximately stoichiometric proportions to directly or indirectly make acetic acid.
The manufacture of acetic acid from carbon monoxide and methanol using a carbonylation catalyst is well known in the art. Representative references disclosing this and similar processes include U.S. Pat. No. 1,961,736 to Carlin et al (Tennessee Products); U.S. Pat. No. 3,769,329 to Paulik et al (Monsanto); U.S. Pat. No. 4,081,253 to Marion (Texaco Development Corporation); U.S. Pat. No. 5,155,261 to Marston et al (Reilly Industries); U.S. Pat. No. 5,672,743 to Garland et al (PB Chemicals); U.S. Pat. No. 5,728,871 to Joensen et al (Haldor Topsoe); U.S. Pat. No. 5,773,642 289 to Denis et al (Acetex Chimie); U.S. Pat. No. 5,817,869 to Hinnenkamp et al (Quantum Chemical Corporation); U.S. Pat. Nos. 5,877,347 and 5,877,348 to Ditzel et al (BP Chemicals); U.S. Pat. No. 5,883,289 to Denis et al (Acetex Chimie); and U.S. Pat. No. 5,883,295 to Sunley et al (BP Chemicals); and EP 845,452-A (Topsoe Haldor. AS) and DE 3712008-A (Linde AG).
The primary raw materials for acetic acid manufacture are, of course, carbon monoxide and methanol. In the typical acetic acid plant, methanol is imported and carbon monoxide, because of difficulties associated with the transport and storage thereof, is generated in situ, usually by reforming natural gas or another hydrocarbon with steam and/or carbon dioxide. A significant expense for new acetic acid production capacity is the capital cost of the equipment necessary for the carbon monoxide generation. It would be extremely desirable if this capital cost could be largely eliminated or significantly reduced.
Market conditions, from time to time in various localities, can result in relatively low methanol prices (an oversupply) and/or high natural gas prices (a shortage) that can make methanol manufacture unprofitable. Operators of existing methanol manufacturing facilities can be faced with the decision of whether or not to continue the unprofitable manufacture of methanol in the hope that product prices will eventually rebound and/or raw material prices will drop to profitable levels. The present invention addresses a way of modifying an existing unprofitable methanol plant to make it more profitable when methanol prices are low and/or gas prices are high.
As far as applicant is aware, there is no disclosure in the prior art for modifying existing methanol plants, including methanol/ammonia plants, to supply stoichiometric MeOH and CO for manufacturing acetic acid, for example, that can be a more valuable product than MeOH.
The present invention involves the discovery that the large capital costs associated with CO generation for a new acetic acid plant can be significantly reduced or largely eliminated by retrofitting an existing methanol or methanol/ammonia plant to make acetic acid. All or part of the syngas is diverted from the MeOH synthesis loop and supplied instead to a separator unit to recover CO2, CO and hydrogen, which are advantageously used in various novel ways to produce acetic acid. The recovered CO2 can be supplied to the reformer to enhance CO production, or to the MeOH synthesis loop to make methanol. The recovered CO is usually supplied to the acetic acid reactor with the methanol to make the acetic acid. The recovered hydrogen can be supplied to the MeOH loop for methanol production, used for the manufacture of ammonia or other products, burned as a fuel, or exported, since the hydrogen is normally produced in excess of the requirements for methanol synthesis in the present invention.
The carbon dioxide can be fed into a catalytic reformer to which natural gas and steam (water) are fed. Syngas is formed in the reformer wherein both the natural gas and the carbon dioxide are reformed to produce syngas with a large proportion of carbon monoxide relative to reforming without added carbon dioxide. Alternatively or additionally, the CO2 can be supplied to the MeOH loop, with additional CO from the synthesis gas and/or additional import d CO2, for catalytic reaction with hydrogen to make methanol.
The syngas can be split into a first part and a second part. The first syngas part is converted to methanol in a conventional methanol synthesis loop that is operated at half of the design capacity of the original plant since less syngas is supplied to it. The second syngas part can be processed to separate out carbon dioxide and carbon monoxide, and the separated carbon dioxide can be fed back into the feed to the reformer to enhance carbon monoxide formation, and/or fed to the MeOH synthesis loop to make methanol. The separated carbon monoxide can then be reacted with the methanol to produce acetic acid or an acetic acid precursor by a conventional process.
Separated hydrogen, which is generally produced in excess beyond that required for methanol synthesis in the present process, can also be reacted with nitrogen, in a conventional manner, to produce ammonia. Also, a portion of acetic acid that is produced can be reacted in a conventional manner with oxygen and ethylene to form vinyl acetate monomer. The nitrogen for the ammonia process (especially for any added ammonia capacity in a retrofit of an original methanol plant comprising an ammonia synthesis loop) and the oxygen for the vinyl acetate monomer process, can be obtained from a conventional air separation unit.
Broadly, the present invention provides, in one aspect, a method for retrofitting an original methanol plant which has at least one steam reformer for converting a hydrocarbon to a syngas stream containing hydrogen and carbon monoxide, a heat recovery section for cooling the syngas stream, a compression unit for compressing the syngas stream, and a methanol synthesis loop for converting at least a portion of the hydrogen and carbon monoxide in the syngas stream to methanol. The method converts the methanol plant into a retrofitted plant for manufacturing a product from carbon monoxide and methanol selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof. The method comprises the steps of: (a) diverting a portion of the syngas stream from at least one reformer to a separation unit; (b) operating the methanol synthesis loop with a feed comprising the remaining syngas stream to produce less methanol than the original methanol plant; (c) operating the separation unit to separate the diverted syngas into at least a carbon monoxide-rich stream and a hydrogen-rich stream, wherein the quantity of hydrogen in the hydrogen-rich stream is greater than any net hydrogen production of the original methanol plant; and (d) reacting the carbon monoxide-rich stream from the separation unit with the methanol from the methanol synthesis loop to form the product, wherein the diversion of the syngas stream is balanced for production of the methanol from the methanol synthesis loop and the carbon monoxide-rich stream from the separation unit for stoichiometric conversion to the product.
Preferably, at least one steam reformer is modified to increase carbon monoxide production in the syngas stream. The syngas stream preferably comprises canon dioxide, and the separation unit produces a carbon dioxide-rich stream that is preferably recycled to at least one reformer to increase the carbon monoxide production.
The reaction step can include the direct catalytic reaction of methanol and carbon monoxide to form acetic acid as in the Mosanto-BP process, for example, or alternatively can comprise the intermediate formation of methyl formate and isomerization of the methyl formate to acetic acid, the intermediate reaction of CO and two moles of methyl alcohol to form methyl acetate and hydrolysis of the methyl acetate ,to acetic acid and methanol, or the carbonylation of the methyl acetate to form acetic anhydride.
In one preferred embodiment of the retrofitting method, the present invention provides a method for retrofitting an original methanol plant that has at least one steam reformer for converting a hydrocarbon/steam feed to a syngas stream containing hydrogen and carbon monoxide, a heat recovery section for cooling the, syngas stream, a compression unit for compressing the syngas stream, and a methanol synthesis loop for converting at least a portion of the hydrogen and carbon monoxide in the syngas stream to methanol. The retrofitted plant can manufacture a product from carbon monoxide and methanol selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof. The retrofitting method comprises the steps of: (a) modifying at least one steam reformer for operation with a feed comprising a relatively increased carbon dioxide content; (b) div rting a portion of the syngas stream from at least one steam reformer to a separation unit; (c) operating the methanol synthesis loop with a feed comprising the remaining syngas stream to produce less methanol than the original methanol plant; (d) operating the separation unit to separate the diverted syngas into a carbon dioxide-rich stream, a carbon monoxide-rich stream and a hydrogen-rich stream; (e) recycling the carbon dioxide-rich stream from the separation unit to at least one modified steam reformer to increase the carbon monoxide formation relative to the original methanol plant and increase the molar ratio of carbon monoxide to hydrogen; (f) reacting the carbon monoxide-rich stream from the separation unit with the methanol from the methanol synthesis loop to form the product, wherein the diversion of the syngas stream is balanced for the production of the methanol from the methanol synthesis loop and the carbon monoxide-rich stream from the separation unit for stoichiometric conversion to the product.
The modified steam reformer is preferably modified to operate at a higher temperature to enhance the carbon conversion to carbon monoxide. The separation unit can include a solvent absorber and stripper for carbon dioxide recovery, and a cryogenic distillation unit for carbon monoxide and hydrogen recovery.
The compression unit preferably has a three-stage compressor, and the syngas stream diversion preferably occurs between the second and third compression stages. The third compressor stage is preferably modified for operation at a lower throughput than the original methanol plant. Where the methanol synthesis loop of the original methanol plant includes a recycle loop compressor, the recycle loop compressor can also be modified for operation at a lower throughput.
The method can also comprise importing a stream of mixed CO/carbon dioxide stream, for example in a 1:2 to 2:1 molar ratio. The imported stream can be supplied to the methanol synthesis loop or to the separation unit, but is preferably supplied to the reformer wherein the carbon dioxide therein is converted to CO.
The method can further comprise the step of reacting the hydrogen in the hydrogen-rich stream with nitrogen to make ammonia. Where the original methanol plant produces a hydrogen-rich stream comprising a loop purge from the methanol synthesis loop that was reacted with nitrogen to make ammonia, the retrofitted plant can use the hydrogen-rich stream from the separation unit as a primary hydrogen source for the ammonia production. With the additional hydrogen available from the syngas, additional ammonia can be produced in the retrofitted plant relative to the original methanol plant.
The method can further comprise installing a vinyl acetate monomer unit for reacting a portion of the acetic acid with ethylene and oxygen to make vinyl acetate monomer. An air separation unit can be installed to make the oxygen for the vinyl acetate monomer unit, and the nitrogen produced from the air separation unit preferably matches the nitrogen required for the additional ammonia production.
In another aspect, the present invention provides a process for making hydrogen and a product selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof, from a hydrocarbon via methanol and carbon monoxide which can be effected by construction of a new plant or retrofit of an existing plant. The process comprises the steps of, (a) catalytically reforming the hydrocarbon with steam in the presence of a minor proportion of carbon dioxide to form a syngas containing hydrogen, carbon monoxide, and carbon dioxide having a molar ratio of R ((H2-CO2)/(CO+CO2)) from 2.0 to 2.9; (b) recovering heat from the syngas to form a cooled syngas stream; (c) compressing the cooled syngas stream to a separation pressure; (d) diverting a major portion of the compressed syngas to a separation unit (e) separating the syngas diverted to the separation unit into a carbon-dioxide-rich stream, a carbon monoxide-rich stream and a hydrogen-rich stream; (f) recycling the: carbon dioxide-rich stream to the reforming step; (g) further compressing the remaining minor portion of the syngas to a methanol synthesis pressure higher than the separation pressure; (h) operating a methanol synthesis loop to convert the hydrogen and carbon monoxide in the further compressed syngas into a methanol stream; and (i) reacting the carbon monoxide-rich stream from the separation unit with the methanol stream from the methanol synthesis loop to make the product. The diversion step is preferably balanced to obtain stoichiometric amounts of carbon monoxide and methanol.
The process preferably has a molar ratio of carbon dioxide to hydrocarbon comprising natural gas in feed to the reforming step from 0.1 to 0.5 and a ratio of steam to natural gas from 2 to 6. The methanol synthesis loop can be operated below a total maximum capacity of the methanol synthesis loop. The process can further comprise the step of reacting the hydrogen in the hydrogen-rich stream with nitrogen in an ammonia synthesis reactor to make ammonia. The process can also comprise the step of separating air into a nitrogen stream and an oxygen stream and supplying the nitrogen stream to the ammonia synthesis reactor. Where the product comprises acetic acid or an acetic acid precursor which is converted to acetic acid, the process can further comprise the step of supplying the oxygen stream from the air separation unit to a vinyl acetate synthesis reactor, along with a portion of the acetic acid from the carbon monoxide-methanol reaction step, and ethylene, to produce a vinyl acetate monomer stream.
In a further aspect the present invention provides a method for retrofitting an original methanol plant, comprising at least one steam reformer for converting a hydrocarbon to a syngas stream containing hydrogen, carbon monoxide, and carbon dioxide, and a methanol synthesis loop for converting hydrogen and carbon monoxide from the syngas stream to methanol, into a retrofitted plant for manufacturing a product from carbon monoxide and methanol selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof. The method includes (1) separating all or part of the syngas stream in a separation unit into respective streams rich in carbon dioxide, carbon monoxide and hydrogen; (2) operating the methanol synthesis loop with a feed comprising (a) carbon dioxide and (b) a portion of the hydrogen-rich stream; and (3) reacting at least a portion of the carbon monoxide-rich stream from the separation unit with methanol from the methanol synthesis loop to form the product. The feed to the methanol synthesis loop can include imported carbon dioxide and/or a portion of the synthesis gas. Preferably, all of the syngas stream is supplied to the separation step. The amount of the hydrogen-rich stream is generally in excess of the stoichiometric hydrogen required by the methanol synthesis loop. Preferably, all of the carbon dioxide-rich stream is supplied to the synthesis loop, and all of the carbon monoxide-rich stream to the reaction step.
In a preferred embodiment, the retrofitting method comprises (1) supplying a major portion of the syngas stream to a separation unit for separating the syngas stream into respective streams rich in carbon dioxide, carbon monoxide and hydrogen, (2) operating the methanol synthesis loop with a feed comprising the carbon-dioxide-rich stream from the separation unit, a minor portion of the syngas stream, and an additional source of carbon dioxide to produce a methanol stream, and (3) reacting the carbon monoxide-rich stream from the separation unit with the methanol stream from the methanol synthesis loop to form the product.
In another preferred embodiment, the retrofitting method comprises (1) supplying the syngas stream to a separation unit for separating the syngas stream into respective streams rich in carbon dioxide, carbon monoxide and hydrogen, (2) operating the methanol synthesis loop with a feed comprising the carbon-dioxide-rich stream from the separation unit, a portion of the hydrogen-rich stream from the separation unit, a minor portion of the syngas stream, and carbon dioxide from an additional source, to produce a methanol stream, and (3) reacting the carbon monoxide-rich stream from the separation unit with the methanol stream from the methanol synthesis loop in stoichiometric proportions to form the product.
In a further preferred embodiment, the retrofitting method comprises importing a stream of mixed CO/carbon dioxide stream, for example in a 1:2 to 2:1 molar ratio. The imported stream can be supplied to the methanol synthesis loop or to the separation unit, but is preferably supplied to the reformer for conversion of the carbon dioxide to CO.
In yet another aspect, the present invention provides a process for making hydrogen and a product selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof, from a hydrocarbon via intermediate methanol, carbon monoxide, and carbon dioxide. The process includes (1) reforming the hydrocarbon with steam to form a syngas containing hydrogen, carbon monoxide, and carbon dioxide, (2) recovering heat from the syngas to form a cooled syngas stream, (3) compressing the cooed syngas stream to a separation pressure, (4) processing the syngas in a separation unit to separate a carbon monoxide-rich stream from the hydrogen and carbon dioxide, (5) operating a methanol synthesis loop to react a first portion of the hydrogen from the separation unit with the carbon dioxide from the separation unit, and additional carbon dioxide from another source, to obtain a methanol stream, (6) reacting the carbon monoxide-rich stream from the separation unit with the methanol stream from the methanol synthesis loop in stoichiometric proportions to form a product selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof.
Regardless of whether the plant is a retrofit or a new plant, where the product comprises acetic acid, the reaction step preferably comprises reacting methanol, methyl formate, or a combination thereof in the presence of a reaction mixture comprising carbon monoxide, water, a solvent and a catalyst system comprising at least one halogenated promoter and at least one compound of rhodium, iridium or a combination thereof. The reaction mixture preferably has a water content up to 20 weight percent Where the reaction step comprises simple carbonylation, the water content in the reaction mixture is more preferably from 14 to 15 weight percent. Where the reaction step comprises low-water carbonylation, the water content in the reaction mixture is more preferably from 2 to 8 weight percent. Where the reaction step comprises methyl formate isomerization or a combination of isomerization and methanol carbonylation, the reaction mixture more preferably contains a nonzero quantity of water up to 2 weight percent. The reaction step is preferably continuous.